1. Field of the Invention
The present invention relates to a depolarization element and a projection type display device, and more particularly, to a depolarization element on which light having coherence is incident and a projection type display device in which a light source having coherence is used.
2. Description of the Related Art
In a projection type display device for displaying projection images on a screen, such as a data projector or a rear projection type television receiver, an ultra-high pressure mercury (UHP) lamp has been used conventionally as a light source. However, a laser has been proposed as a light source from the viewpoint of superiority in monochromaticity and long service life.
In addition, since the wavelength band in the vicinity of 645 nm, i.e., the wavelength of red, has a broad spectrum because of the nature of the UHP lamp, a combination light source in which a laser is used as a red light source and a UHP lamp is used for blue and green wavelength bands has also been proposed. However, such a projection type display device in which a laser is used as a light source has a problem that granular speckle noise due to the coherence of laser light occurs in projection images and degrades the quality of the projection images.
Hence, a projection type display device, in which a phase modulation element formed of liquid crystal or polymer liquid crystal is disposed as means for removing speckle noise in an optical path from a laser light source, is disclosed by WO/2008/047800.
Furthermore, an example of this phase modulation element is given in which one or both of the orientation direction of a slow axis and a retardation value are distributed while having different values from each other inside a plane perpendicular to the optical axis of the laser light, and the orientation direction of the slow axis is distributed in a radial direction or a circumferential direction around the optical axis over the entire plane on which the laser light is incident.
However, in the phase modulation element according to WO/2008/047800, the orientation direction of the slow axis and the retardation value are distributed so as to change totally smoothly on the plane perpendicular to the optical axis of the incident laser light. Hence, with respect to the distribution in some narrow regions in the regions of the phase modulation element on which the light is incident, the changes in the orientation direction of the slow axis and the retardation value are small, whereby there occurs a problem that it is difficult to obtain depolarization in uniform distribution and it is difficult to reduce speckle noise partially and significantly in the projection type display device. Furthermore, in the case that a setting is done to obtain high depolarization for a light beam having a specific wavelength among light beams incident on the phase modulation element, depolarization equivalent thereto cannot be obtained for a light beam having a wavelength different from the specific wavelength. Hence, there is a problem that depolarization has wavelength dependence.